Ammeters are often a sub component of electrical measurement products including digital multi-meters (DMMs) and source measure units (SMUs). There are several ways in which the current through a device under test (DUT) may be measured. SMUs are generally used to make precision measurements in many fields, including the testing of semiconductor products. Typical SMU designs include a voltage or current source with integrated voltage and current measurement capabilities. A device under test (DUT) is typically coupled to the SMU and then stimulated with either the voltage or current source.
There are several ways in which the current through a DUT may be measured. For example, a shunt ammeter may be used to simply sense the voltage across a resistor RS. RS must be kept small to not cause a large burden voltage to the input signal. A low noise gain stage is typically required to amplify the burden voltage so it can be measured.
One of the most common ammeters is the feedback ammeter. FIG. 1 illustrates an example of a typical feedback ammeter 100 configured with a high gain operational amplifier (op-amp) A to pull an input 102 through a resistor RS. The op-amp A keeps the burden voltage low because of its high DC gain (e.g., typically greater than 1 million). This allows RS to be larger, thus allowing the output signal 104 to be larger.
However, a significant problem with the feedback ammeter is that it is generally prone to instability with capacitive loads. A variation of the feedback ammeter called an active shunt has been developed, in which the input-impedance is resistive from DC to the bandwidth of the op-amp, ω1.
An active shunt ammeter configuration generally replaces the op-amp used in the feedback ammeter with a fixed gain amplifier. The result is a gain that is constant to higher frequencies. At the frequency the amplifier begins to roll off, the capacitor impedance (1/jωCs) is designed to have a magnitude that equals RS. The roll off of the parallel impedance of RS and Cs combined with the roll off the amplifier's gain, results in an input-impedance of the ammeter that is constant across the entire bandwidth of the amplifier. The result is a shunt like ammeter with higher output signal vs. burden voltage than a traditional shunt ammeter and none to the stability issues of feedback ammeters.
FIG. 2A illustrates an active shunt ammeter design 200 using a controlled negative gain across a parallel RC feedback element 202 such that input impedance of the circuit is a resistance equal to the R divided by the gain. In this example, the active shunt ammeter 200 includes a fixed gain differential amplifier 208 with a parallel resistor 204 and capacitor 206 connected between the negative-input and output terminals of the fixed gain differential amplifier 208. The RC product of resistor 204 and capacitor 206 is selected to equal to the amplifier's gain-bandwidth divided by the fixed gain.
FIG. 2B is a graph showing the gain B(s) of the fixed gain amplifier 208 as well as other parameters. In general, the gain B(s) (shown by reference number 250) of fixed gain amplifier 208 remains essentially constant from DC until a target frequency 252. Once the target frequency 252 is reached, the gain B(s) of the fixed gain amplifier 208 rolls off, e.g., at 20 db per decade. In this example, the operational amplifier 210 in FIG. 2A has a gain A(s) that is much higher than B(s). However, operational amplifier 212 functions as an inverter in the feedback path yielding the composite gain B(s) for the fixed gain amplifier 208. This configuration provides a controlled negative gain across the parallel RC feedback element 204, 206 such that the input impedance of the circuit is a resistance equal to the RS divided by the gain.
In FIG. 2A, ωt is the gain bandwidth of the operational amplifier 210. Also shown in FIG. 2A is the resistance of resistor 204 (Rs) which remains constant over the frequency range shown. Also shown in FIG. 2A is the input impedance Zin of the active shunt ammeter 200. In general, the input impedance Zin configured to be significantly less than RS and to appear to be resistive in nature for frequencies less than and equal to ωt. In this example: Zin=Rs*(R1/(R1+R2)) and Cs˜R2/(ωt*Rs*R1).
If the feedback element 202 was resistive only, i.e., if capacitor 206 was omitted, the input impedance Zin would increase with frequency after the target frequency 252. The impedance of capacitor 206 may be selected to equal the impedance of the resistor at the target frequency 252. This causes the impedance of the feedback element 202 to drop at the same frequency the operational amplifier 210 begins to roll off. This configuration yields a flat input impedance that does not increase after the target frequency 252 as shown in FIG. 2B.